1. Field of the Invention
The present invention relates to a photocatalyst layer having a decomposing function such as deodorizing, antibacterial and soil-preventing actions and also a hydrophilic function, and a photocatalyst coated composition having said photocatalyst layer, more particularly relates to a photocatalyst layer having a reactiveness to electromagnetic wave whose wavelength is longer than that of ultraviolet rays and a photocatalyst coated composition having said photocatalyst layer.
In the specification, xe2x80x9celectromagnetic wavexe2x80x9d includes light of gamma rays, ultraviolet rays, visible rays and infrared rays or the like and electric radiation.
2. Description of the Prior Art
Up to now, since photocalytic material such as titanium dioxide (TiO2) (hereinafter called xe2x80x9ctitaniaxe2x80x9d in the specification) or the like shows excellent decomposing function and hydrophilic function by being bathed in ultraviolet rays included in sunlight or the like. Therefore, such a metal oxide layer has been utilized conventionally in many fields as photocatalyst.
Among the photocatalyst materials, said titania will be described as an example.
When titania is bathed in ultraviolet rays included in sunlight, electron and holes are generated on a titania surface. Then the electron reduces oxygen in air then changes the oxygen into super oxide ions (O2), or, holes oxidize water adhered on the titania surface then changes the water into hydroxyl group radical (OH). The super oxide ions and the hydroxyl group radical decompose an organic compound such as dirt on the titania surface with oxidizing. That is, photocatalysis excites catalysis on titanium oxide by reduction power of the electron and the oxidization power of holes.
To describe now the hydrophilic function, the super oxide ions and hydroxyl groups generated by the ultraviolet irradiation as described above decompose hydrophobic molecules present on the titania surface to produce hydroxyl groups, and the atmospheric moisture is adsorbed by the hydroxyl groups thus produced to form a thin water film, thus imparting hydrophilicity to the titania surface. Accordingly, photocatalysts are frequently utilized in lenses, interior materials and furniture, such as mirrors, wall papers and curtains for the purpose of imparting deodorizing, antibacterial and soil-preventing actions, because of their hydrophilic function as well as the decomposing function.
When these photocatalysis are to be utilized in products such as interior materials and furniture, the product is impregnated with titania as a major component of the photocatalyst and is irradiated well with ultraviolet light. As a technique of achieving this, it is practiced to form a titania layer on the surface of a product to be treated.
As one method of forming a titania layer, a product to be treated made of titanium or titanium alloy is oxidized on the surface to form an oxide layer or a titania layer utilizing its liability to undergo oxidation reactions, since titanium per se is an active metal and have particularly great affinity with oxygen.
As other methods for forming a titania layer, a sol-gel method and a binder method are employed.
According to the sol-gel method, an organic titanium sol such as of titanium alkoxide and titanium chelates, which are precursors of titania, is applied onto the surface of a product to be treated having heat resistance, such as glass and ceramics by means of spray layer and the like, and then dried to cause gelation, followed by heating to 500xc2x0 C. or higher and form a stiff titania layer. Since titania particles are distributed over the entire surface of the product to be treated, the titania layer thus formed has high decomposition power and high hardness.
Meanwhile, according to the binder method, titania particles are immobilized on the surface of a product to be treated using a binder, for example, an inorganic binder such as silica or an organic binder such as silicone. The difference of the binder method from the sol-gel method is that the heating temperature may be the hardening temperature of the binder, so that the former requires a heat treatment at about 100xc2x0 C. or lower and no high-temperature treatment.
The conventional photocatalyst layer methods described above involve the following problems.
1. Difficulty in forming a titania layer on a surface to be treated
(1) The method of forming a titania layer by surface-oxidizing a product to be treated made of titanium or titanium alloy involves problems in that titanium per se is expensive to cause cost elevation and that titanium has poor processability and the fields of its application are limited.
(2) The sol-gel method also involves a problem in that it requires a heat treatment at about 500xc2x0 C. or higher for converting the organic titanium such as titanium alkoxide and titanium chelates, which are precursors of titania, into a titania layer, so that the product to be treated should have heat resistance, and that the product to be treated is limited to glass, ceramics and the like. If a titania layer is to be formed on the surface of a metal according to the sol-gel method, the metal surface is oxidized by the high-temperature heat treatment to cause reduction of commercial value due to deterioration and reduced luster.
The sol-gel method further involves a problem in that it requires much time and labor since the organic titanium is applied many times, that it requires an expensive equipment to cause cost elevation, and that harmful waste is by-produced.
(3) Meanwhile, the binder method can solve the problems inherent in the sol-gel method and enjoys merits in that it can treat various kinds of products and that it is relatively inexpensive. However, it involves a problem in that it is necessary to use as the binder a material which has high adhesion with the product to be treated and which is not susceptible to the decomposing function of the photocatalyst, and that selection of binder influences the effect of the catalyst.
Further, the titania layer formed according to the binder method has a hardness lower than that of the layer formed according to the sol-gel method, disadvantageously. In order to increase the hardness of the titania layer to be obtained according to the binder method, the amount of binder is increased to enhance adhesion. In this case, however, the amount of titania is reduced relative to the binder, and the titania layer shows poor decomposing power. On the contrary, if the amount of binder is reduced, the amount of titania to be exposed on the surface of the product to be treated is increased to show enhanced decomposing power, but the adhesion is reduced to readily cause peeling of the titania layer, resulting in the reduced hardness, disadvantageously. 2. Not reacting to an electromagnetic wave whose wavelength is longer than that of ultraviolet rays.
In sunlight poured on the earth, only about 4 to 5% of ultraviolet rays is included, the other is composed of about 50% of infrared rays and about 45% of visible rays. In addition, sunlight is maximized in the neighborhood of 450 nn, i.e., a visible radiation area, therefore, if photocatalyst responding to light of wavelength in the visible radiation area can be provided, more effective photocatalytic reaction can be obtained.
However, said photocatalyst made of titania is excited only by ultra violet rays whose wavelength is shorter than 380 rim and if visible rays whose wavelength is about 400 to 800 nm or infrared rays whose wavelength is over 800 nm is irradiated, photocatalysis is not shown. Thereby, visible rays and infrared rays comprising of the greater part of sunlight can not be used for decomposing injurious material or the like, therefore, sunlight can not be used efficiently.
Furthermore, in a room or the like which can not be irradiated by sunlight, ultraviolet rays are not irradiated, thereby, said photocatalyst can not be used. In addition, if said photocatalyst is used in the room, a special light source such as a bactericidal lamp emitting an ultraviolet rays must be prepared, therefore a use of photocatalyst material is restricted.
On the other hand, photocalytic material such as cadmium sulfide (CdS) or cadmium selenium (CdSe) or the like can show catalysis by light of wave length in the visible radiation area, however, a phenomenon of which cadmium sulfide (CdS) or cadmium selenium (CdSe) is ionized then dissolved by electrochemical reaction such as transfer of electron in case of reaction. Thereby, the photocalytic material can not be used stably.
To improve reactive efficiency of photocatalyst material such as titanium dioxide or the like as previously mentioned, the following methods are suggested, however, in case of using the methods, some problems still remain as described below;
2-1. It is known that photocatalysis of a titania layer is improved by supporting precious metal such as platinum, palladium, gold, silver and copper or the like on a metal oxide layer, however these precious metals are expensive, therefore if 1% of the precious metal to a weight of a formed metal oxide layer is supported, cost for forming the layer is rose 5 to 10 times.
2-2. It is known that absorption of light can be shifted to a side of long wave length and light in a visible radiation area can be absorbed by implanting a very small quantity of metal ions such as vanadium (V), chrome (Cr) or the like in a layer. However, in such case, facilities for implanting the metal ions is expensive and a large amount of initial investment is required.
2-3. In addition, there is a method for improving efficiency of photocatalytic reaction by securing a specific surface area of which a layer is formed as large as possible, however, the more the specific surface area is enlarged, the more inorganic material can be adhered easily by being the surface uneven. The photocatalyst comprising of metal oxide can decompose organic material, however it can not decompose inorganic material, therefore, adhered inorganic material is kept adhering on a surface using a photocatalytic agent without being decomposed, thereby photocatalysis can not be obtained.
The invention has been made as a solution to the above problems, and an object thereof is to obtain a metal oxide layer which can be formed on a surface of a product to be treated by relative simple method such as shot peening and to provide a metal oxide layer showing photocatalysis by responding not only to ultraviolet rays but also to an electromagnetic wave whose wave length is longer than that of ultraviolet rays, i.e., visible rays or the like, for example, visible rays, infrared rays and electric wave or the like.
To achieve the above objective, a metal oxide layer of the present invention is a layer having oxide deficit tilting structure comprising of metal oxide formed by shot peening. The layer having the oxygen deficit structure comprising of metal oxide can be formed on a surface of a product comprising of metal, ceramics or a mixture of them.
In addition, precious metal may be supported in said layer, and/or metal ions can be implanted in said layer.